Software updates and TUF

The Root Key

As mentioned previously, the root key or keys must be well protected, because these are the keys you use to sign the other keys, which are then used to sign the software, update information, and so on. Because you only need to use the root keys to sign other keys, it is relatively simple to keep them on an offline system. For example, you could use a cheap netbook with WiFi/Ethernet disabled, and move keys to and from it using a USB stick. If you want even more security, you can use a hardware module to contain the key. These tend to be quite expensive, but because TUF uses GPG, you can use an OpenGPG cryptocard [6], which you can generally order online for less than US$ 20 and which supports 4,096-bit RSA keys.

The Targets Role

The targets role is essentially used to sign the files that are being offered for download. The targets role can be automated; for example, you can set up a server to sign uploaded files automatically. This, of course, should be done on a secured, dedicated server; you shouldn't also use it to offer the files for download, for example. However, if you use different keys for metadata signing, then a compromise of the target role key can't exactly be used to push updates. The attacker will be able to sign the files but not modify the metadata telling the client to install them.

The Delegated Targets Role

The delegated target role is optional; basically, instead of using a single role to sign files, you can delegate it. For example, you may have a main application with user-contributed plugins, and the signing of these plugins could be delegated to other target roles. This approach allows multiple organizations to participate in an update system securely. You can sign the third-party key, and they can then use it to sign their files and send them to the update system.